Dispersion processes for polymerizing fluoro olefin monomers in aqueous media employ a surfactant to provide stability to the aqueous dispersion of particles of the resulting fluoropolymer. Different surfactants are chosen for use in dispersion polymerization because of their influence on reaction rate, dispersed fluoropolymer particle size, dispersion stability, color and the like.
Fluorosurfactants used in the polymerization are usually anionic, non-telogenic, soluble in water and stable to reaction conditions. The fluorosurfactants as disclosed in U.S. Pat. No. 6,774,164 contain perfluoroalkyl groups having 4 to 18 carbon atoms. It is also known that the presence of a fluorocarbon “tail” in the hydrophobic segment of surfactants provides extremely low surface energy. Such fluorinated surfactants are much more surface active than their hydrocarbon counterparts. For surfactants and surface treatment agents with fluorochemical chains, longer perfluoroalkyl chains contain a higher percentage of fluorine at a given concentration and typically provide better performance. However, the fluorinated materials derived from longer perfluoroalkyl chains are more expensive.
Honda et al, in Macromolecules, 2005, 38, 5699-5705 teach that for perfluoroalkyl chains of greater than 8 carbons, orientation of the perfluoroalkyl groups, designated Rf groups, is maintained in a parallel configuration while for such chains having less than 6 carbons, reorientation occurs. This reorientation decreases surface properties such as contact angle. Therefore, it is desirable to reduce the fluorine content with delivery of the same or higher performance.
It is desirable to provide new and improved fluorinated surfactants in which the perfluoroalkyl group of the prior art is replaced by partially fluorinated terminal groups which show increased fluorine efficiency. By “fluorine efficiency” is meant the ability to use a minimum amount of fluorochemical to obtain a desired surface effect. A surfactant having high fluorine efficiency generates the same or greater level of surface effect using a lower amount of fluorine than a comparative surfactant. The present invention provides such improved fluorinated surfactants.